(a) Field of the Invention
The present invention relates to an orthogonal frequency division multiplexing (OFDM) system. More specifically, the present invention relates to an automatic gain control device and method of the OFDM system.
(b) Description of the Related Art
Conventionally, an automatic gain control (AGC) process and a DC offset calculation process have been sequentially performed in the training symbol interval in the OFDM system, which will now be described.
FIG. 1 shows a configuration diagram of the preamble of the IEEE 802.11a WLAN.
As shown, the preamble has short training sequences and long training sequences. The short training sequences are used to detect signals, perform AGC, and estimate coarse frequency offsets, and the long training sequences are used to acquire fine symbol synchronization and estimate fine frequency offsets.
The training symbols are normalized, but data symbols are generated by performing inverse fast Fourier transform on data, and hence, their energy is not constant, and the AGC is performed by using the training symbols in the preamble interval.
An AGC device of the general OFDM system finds energy of input I and Q data, takes an average thereof, converts the average into a value [dB] to be compensated by the AGC device, feeds the difference between the converted value and a reference value back to the AGC device, and thus controls the gain in the training symbols. An important parameter in the above-noted AGC process is a reference power, and the bit error rate (BER) of the OFDM system can be reduced by controlling the reference power and controlling the gain. That is, variations of the reference power improve or deteriorate the total performance of the OFDM system.